No one was injured in the incident, which occurred at 5 a.m. local time (9 p.m. ET Wednesday), but the top of the submarine's periscope was shorn off and the sub was forced to use a second periscope, the official said.

The reactor on the nuclear-powered USS Jacksonville was not affected. "There was no damage to the propulsion plant systems and there is no concern regarding watertight integrity," the Navy's Bahrain-based Fifth Fleet said in a statement.

As for the trawler, it may not have even realized it hit the submarine and did not appear to suffer any damage.

The vessel "continued on a consistent course and speed offering no indication of distress or acknowledgment of a collision," the Fifth Fleet said in a statement.

A Navy investigation is underway.

In the past, Iran has threatened to close the Strait of Hormuz over its quarrel with United States about its nuclear ambitions.

The U.S. says it keeps a naval presence in the region to maintain security.